The invention relates to a method of adjusting a given pretension in threads on tensile strength testing apparatus and the like.
Testing methods on automatic tensile strength testing apparatus are extensively standardized, i.e., the starting conditions, under which a test is carried out, are determined by recognized instructions in order to obtain the same preconditions for all tests. In addition to the clamping length, deformation velocity, moisture and temperature of the surrounding atmosphere among other factors, the pretension which the thread should exhibit at the start of the test is also a determined value, since the stress-strain values are greatly dependent on this pretension. Also, the thread should be able to be clamped when it is stretched.
A known tensile strength testing apparatus produces, for example, a certain pretension of the inserted thread by means of a calibrated thread brake over which the thread is passed between a supply bobbin and a transport clamp of the insertion mechanism. By correspondingly adjusting the braking force of the thread brake, the thread is inserted into the testing clamps under this tension. After closing the clamps, the stretching process commences at this pretension.
Other arrangements use a yarn balance for the thread to be inserted, so that the desired pretension is imparted to the thread.
However, these known arrangements have various disadvantages. Thread brakes exert a braking force on the thread which can only be defined with difficulty, and it is difficult to achieve a change in the thread tension; circumstances permitting, auxiliary devices have to be used to determine the actual thread tension. Deflection eyelets or the like of such auxiliary devices may also falsify the measuring result. Yarn balances are also very costly and are complicated mechanisms which require a constant or periodic maintenance.